空穴主导雪崩倍增的短波长SiC紫外单光子探测器

Short-Wavelength SiC Ultraviolet Single Photon Detector Based on Hole-Dominated Avalanche Multiplication

制备并分析了SiC nip雪崩光电二极管(APDs)雪崩倍增的物理机制。与280 nm紫外光相比,在240 nm紫外光入射时,SiC nip APD表现出更高的增益和更大的单光子探测效率。在240 nm紫外光入射时,SiC nip APD表现为空穴主导碰撞离化过程,随着入射光波长增加到280 nm,电子和空穴共同主导碰撞离化过程。由于SiC中空穴的碰撞离化系数大于电子,空穴主导的碰撞离化过程将具有更大的光子雪崩概率和更高的增益。因此,得益于空穴主导雪崩倍增过程,SiC nip APD更适用于短波长紫外光探测。

Objective SiC avalanche photodiodes(APDs)have the merits of visible light blindness,low weight,and high gain,exhibiting obvious advantages in weak ultraviolet detection.SiC APDs are mostly designed as pn junction structures,which can be divided into pin and nip APDs,according to the distribution of the epitaxial layers.When the SiC APD is conducting weak light detection,the device works under the avalanche state;the SiC pin and nip APDs exhibit completely opposite carrier drift directions.Holes in the pin APD drift toward the p-type layer,that is,the device surface,whereas the electrons drift toward the n-type layer,that is,the substrate;the nip APD exhibits the opposite.Cha et al.estimated the absorption coefficient of SiC at room temperature,and the results showed that SiC has a higher absorption coefficient for high-energy photons.Therefore,for the SiC pin and nip APDs,owing to the difference of the carrier drift direction,the types of carriers that cause collision ionization may be different for different wavelength detections.Considering that the collision ionization coefficient of holes in SiC is larger than that of electrons,different types of collision ionization carriers will certainly affect the detection performance of APDs.In this work,SiC nip APDs are designed and fabricated with a higher single photon detection efficiency for short wavelength ultraviolet light,benefitting from the hole dominated collision ionization process.This work is conducive for the in-depth understanding of the working mechanism of SiC APD and provides theoretical guidance for its optimization in the future.Methods The SiC nip APD is fabricated on an n-type 4H-SiC substrate(Fig.1).The epitaxial structure is a 10μm thick p-type contact layer(acceptor doping concentration NA=3×1018 cm-3),a 0.05μm thick transition layer,a 0.7μm thick n-type avalanche multiplication layer(donor doping concentration ND=1×1015 cm-3),a 0.2μm thick n-type transition layer(ND=1×1018 cm-3),and a 0.15μm thick n-type contact layer(ND=1×1019 cm-3)from bottom to top.The first step of APD fabrication is mesa etching.To suppress the peak electric field at the edge of mesa,the device is designed to be a beveled mesa structure(beveled angle of∼5°),and the etching depth is 0.5μm.The epitaxial wafer is then passivated by the thermal oxidation layer and the plasma-enhanced chemical vapor deposition SiO2 layer.Finally,both the front and back contact metals(Ni/Ti/Al/Au)are deposited by electron-beam evaporation and annealed by rapid thermal annealing at 850℃for 3 min in N2 ambience.Results and Discussions To study the avalanche mechanism of SiC nip APD,the gain curves of SiC nip APD are plotted(Fig.3).When the wavelength of the incident ultraviolet light increases from 220 nm to 320 nm,the breakdown voltage of the nip APD increases by 1.3 V.When the over bias is 3 V,the dark count rate is 1.7 Hz/μm2,and the single photon detection efficiency of the SiC nip APD for 240 nm and 280 nm incident UV light is 11.4%and 6.5%,respectively(Fig.5).In the same working conditions,the single photon detection efficiency of the SiC nip APD for short wavelength ultraviolet light is evidently higher.The quantum efficiency of the SiC nip APD under 240 nm and 280 nm incident UV light is 15%and 45%,respectively(Fig.6).Assuming that the SiC nip APD operates at 3 V overbias,the estimated photon avalanche probability is 84%and 16%at 240 nm and 280 nm illumination,respectively.It can be observed that the high single photon detection efficiency of the SiC nip APD for high-energy ultraviolet light is mainly related to the large photon avalanche probability.When the 240 nm ultraviolet light is incident on the SiC nip APD surface,photogenerated carriers are generated on the device surface and enter the depletion region through diffusion.Under the avalanche state,when the hole enters the depletion region,it drifts towards the substrate under the strong electric field,and its acceleration distance and collision ionization process span the entire depletion region.When the electrons drift towards the device surface after entering the depletion region,the acceleration distance and collision ionization process can be ignored.Thus,at 240 nm illumination,the avalanche process of the SiC nip APD is dominated by holes.With the increase of the ultraviolet light wavelength,the contribution of the electrons gradually increases.The collision ionization coefficient of holes in SiC is larger than that of electrons;therefore,the hole-dominated collision ionization process exhibits a higher avalanche gain and higher single photon detection efficiency.Conclusions In this work,SiC nip APDs are designed,and their avalanche characteristics are studied in detail.Under high energy ultraviolet light illumination,the avalanche multiplication process of the SiC nip APD is dominated by holes.With the increase of the incident light wavelength,the contribution of the electrons gradually increases.As the collision ionization coefficient of holes in SiC is larger than that of electrons,the SiC nip APD exhibits a higher gain and higher single photon detection efficiency for high-energy ultraviolet light illumination.The essential characteristics of the SiC nip APD ensure that the device is more suitable for short wavelength ultraviolet detection.